1. Field of the Invention
The present invention relates to novel processes for producing hydrolyzed protein. More specifically, the present invention relates to processes for easily producing high-quality, highly stable hydrolyzed protein useful for seasonings or the like by hydrolyzing a protein-containing starting material by an action of an enzyme. The present invention further relates to processes for preparing food products by incorporating hydrolyzed protein prepared by such a process in a food product.
2. Discussion of the Background
Various processes are already known for producing hydrolyzed protein from a protein-containing starting material by an enzyme on an industrial production scale.
For example, Japanese Patent Kokai Publication JP-A-51-35,461 discloses a process for producing a liquid seasoning by reacting denatured de-fatted soybeans with an alkaline protease and a peptidase, and Japanese Patent Kokai Publications JP-A-6-125,734, JP-A-9-75,032, and JP-A-9-121,807 disclose a process for producing a seasoning by hydrolysis of various proteins with a protease and a peptidase contained in a culture of a koji mold.
Further, a process for producing a seasoning having a high content of glutamic acid (see Japanese Patent Kokai Publication JP-A-2000-88) or an amino acid mixture with reduced browning (see Japanese Patent Kokai Publication JP-A-2000-14,394) by using a microbial culture obtained by a specific incubation method or by using a specific condition of hydrolysis of a protein-containing starting material have also been reported.
However, such conventional techniques are problematic in that during the production of a hydrolyzate by enzymatic hydrolysis of a starting material containing a solid protein, the quality and the yield of the hydrolyzed protein obtained are decreased due to the growth of microorganisms other than the microorganism used as an enzyme source, so-called contaminants, in the hydrolysis step. In order to solve this problem, bacteriostatic substances such as alcohols, sodium chloride, ethyl acetate, and the like have been employed in the hydrolysis step in the conventional processes (see the above Japanese Patent Kokai Publications JP-A-6-125,734 and JP-A-9-75,032).
However, in these processes, an additional step of separating and removing bacteriostatic substances after the hydrolysis step was required. Especially when sodium chloride was employed as a bacteriostatic substance, it was quite difficult to decrease sodium chloride to less than an appropriate concentration without deteriorating the quality of the resulting hydrolyzed protein. Moreover, it was almost impossible to prevent the occurrence of a so-called brewing odor or soy sauce odor in the hydrolyzed protein obtained by hydrolysis in the presence of bacteriostatic substances. As a result, the range of utilities of the resulting hydrolyzed protein was extremely restricted.
Further, in the conventional processes, attempts were naturally made to subject a starting material containing a solid protein to hydrolysis after removal or sterilization of contaminants incorporated in the protein-containing starting material or a microbial culture as an enzyme source. It is relatively easy to perform hydrolysis of a protein-containing starting material after sterilization thereof on a laboratory scale. However, in industrial mass-production, the prevention of microorganism contamination in the sterilization step and the hydrolysis step is very difficult to perform.
In the commercial production of a liquid seasoning of an enzymatically hydrolyzed protein, it is important to prevent microorganism contamination in view of quality control. The main contamination sources include the protein-containing starting material, the enzyme preparation or the enzyme-containing fermentation broth and the production equipment. In the step of sterilizing a protein-containing starting material prior to the enzymatic hydrolysis, bacteriostatic agents such as alcohols, sodium chloride, ethyl acetate and the like have been employed as stated above. However, incorporation of these bacteriostatic agents into products limits the use of the products. Further, the additional step of removal of such bacteriostatic agents incurs the problem of increased production cost.
With respect to the enzyme preparation or the enzyme-containing fermentation broth, a filtrate of the enzyme solution with an absolute filter or an aseptically fermented enzyme broth is appropriate for the aseptic use in the subsequent hydrolysis process. For the sterilization of the production equipment, there are treatment methods depending on characteristics of the equipment. Washing with an alkaline washing solution or an acid washing solution and steam heating are available and may be performed at a low cost.
Accordingly, the most serious problem among others is to completely sterilize a protein-containing starting material without the use of sodium chloride or alcohols. In particular, the sterilization of a protein-containing starting material containing solid matter involves a lot of difficulties as follows. Looking at a typical method, in which a pulverized protein is dispersed in water and the dispersion is heat-sterilized, it is first found that the dispersion contains solid matter, the inside of which is hardly heated by ordinary heating methods. Further, when the protein concentration in the dispersion is increased, the viscosity of the dispersion becomes quite high, and equipment having a very high power for feeding the dispersion is required, which increases the production cost. In particular, a protein-containing starting material such as wheat gluten includes active gluten which is easy to bind when its powder is dispersed in water. Thus, it cannot be evenly dispersed. Moreover, even if the dispersion can be fed smoothly, the protein ingredients are denatured and solidify inside the unit for the sterilization step, such as a plate-type heat exchanger or a nozzle-type heater, to clog the same.
As one technique to partially solve this problem, a method has been proposed in which a protein-containing starting material is finely pulverized to a diameter of not more than 300 xcexcm and dispersed in hot water of not less than 80xc2x0 C., thereby improving the dispersibility of the protein-containing starting material and preventing bubbles from being incorporated into the dispersion at the same time (see Japanese Patent Kokai Publication JP-A-11-313,693). In this method, the dispersing of the protein-containing starting material which was difficult in the past is enabled for protein dispersions having a relatively low concentration. Further, the viscosity of the dispersion is decreased, and bubbles are not incorporated into the dispersion, whereby complete sterilization is enabled in the subsequent sterilization step.
However, in order to decrease the production cost in commercial production, it is necessary to increase the concentration of the protein dispersion and to improve the equipment productivity in the subsequent hydrolysis step. None of the prior techniques are satisfactory for sterilizing such a protein dispersion having a high concentration.
That is, as the concentration of the protein dispersion is increased, the protein-containing starting material is hardly dispersed uniformly. Moreover, when the protein-containing starting material is aggregated in an aqueous medium, it contains air and the heating is not uniform so that complete sterilization is difficult in this state. Further, even though the protein-containing starting material can be dispersed, the viscosity of the dispersion is high, and accordingly, it is difficult to feed for the subsequent step. In theory, it is possible to disperse the protein-containing starting material with a high-performance dispersion vessel and feed the dispersion with a pump for a highly viscous liquid. However, such production equipment is expensive.
With respect to the sterilization step, there are two methods, a direct heating method and an indirect heating method. In the direct heating method, a heat medium and a subject (such as the dispersion of the protein-containing starting material) to be treated are directly contacted with each other to heat the subject. A direct heating-type ultra high temperature instantaneous sterilizer for high viscosity is available. According to this method, even a food having a high viscosity of several hundreds of thousands of centipoises [c.p. (mPaxc2x7s)] can be heated in a short period of time by being mixed with a steam as a heat medium at good efficiency, and completely sterilized by maintaining the temperature for a predetermined period of time. Moreover, the heated subject can be subjected to reduced pressure to evaporate the steam charged in the heating and instantaneously cooled to approximately the original temperature. Nevertheless, since the steam as the heat medium is directly mixed with the food, the use of chemicals such as boiler compounds and the like in the water supply to the boiler is limited, and it is necessary to further add a step of producing steam which is suitable for use in the food. In addition, there is the problem that the method cannot be applied to a subject which contains solid matter having a size of several millimeters.
Meanwhile, in the indirect heating method, a subject to be treated is indirectly heated with a heat medium via a heat transfer member. The indirect heating method has been often used in the sterilization of highly viscous food in the food industry because of the simple mechanical structure, good operability, and low-cost apparatus. In this connection, a tubular system or a scratch-type sterilizer is a typical indirect heating-type sterilizer for highly viscous foods. The tubular system is a system in which a highly viscous food is passed through a tube and heated from the outside with hot water or the like. Since the pressure supplied is increased owing to the pressure drop in a pipe, its length is limited, so that plural pipes are sometimes arranged in parallel to give a multi-pipe system. Advantageously, the structure is simple, and the system is easy to assemble or disassemble and is also easy to wash. On the other hand, the heat transfer surface is liable to scorch. And the fluid flowing there inside is hard to mix, and thus the temperature tends to be non-uniform. Accordingly, this method is inappropriate when strict control of temperature is required.
The scratch-type heat exchanger is designed to offset the defect of the tubular system, and scratches off the food near the heat transfer surface with a vane to prevent scorching. At the same time, the food in the tube can be stirred for thermal uniformity. However, the scorching on the heat transfer surface still cannot completely be prevented for almost all highly viscous foods.
After a fixed time of operation, the heat exchanger is usually washed in the assembled state. However, in the case of hard scorching, the heat exchanger has to be disassembled and washed. These indirect heating-type sterilizers are still problematic in the heat transfer, and sometimes cause thermal deterioration of qualities (degradation of nutrients, taste, color and the like) owing to a long heating time and mechanical change (physical change in viscosity and the like) caused by stirring.
Moreover, in the fermentation industry in which a complete sterilization step of a culture medium is indispensable, an indirect heating plate-type heat exchanger using steam as a heat medium is generally used for sterilizing a culture medium. However, when heat sterilization is conducted by simply passing a protein dispersion having a high concentration through such a heat exchanger, the protein dispersion is thermally denatured, a decrease in heat transfer efficiency and an increase in pressure drop due to scaling in the heater or the cooler occur, and in the worst case, it becomes impossible to pass the dispersion.
Thus, there remains a need for an improved method for performing the complete sterilization of a highly viscous protein dispersion having a high concentration on an industrial scale, which does not depend on a special technique or equipment.
Accordingly, it is one object of the present invention to provide novel methods for preparing hydrolyzed protein.
It is another object of the present invention to provide novel methods for sterilizing or rendering aseptic a protein-containing starting material for the production of hydrolyzed protein.
It is another object of the present invention to provide novel methods for sterilizing or rendering aseptic an aqueous dispersion of a protein-containing starting material for the production of hydrolyzed protein.
It is another object of the present invention to provide novel methods for sterilizing or rendering aseptic a highly concentrated aqueous dispersion of a protein-containing starting material for the production of hydrolyzed protein.
It is another object of the present invention to provide novel methods in which an aqueous dispersion of a protein-containing starting material, especially, a dispersion having a high concentration can easily be made substantially aseptic in the production of an enzymatically hydrolyzed protein on an industrial scale without introducing special equipment, thereby improving the equipment productivity and the product quality and widening the use range of products.
It is another object of the present invention to provide novel methods for preparing food products by adding a hydrolyzed protein so produced to a food product.
These and other objects, which will become apparent during the following detailed description, have been achieved by the inventors"" discovery that an aqueous dispersion of a protein-containing starting material can be made substantially aseptic by being heated and maintained while under an acidic condition with a plate-type heat exchanger using a liquid as a heat medium. This finding has led to the completion of the present invention.
That is, a process for producing a hydrolyzed protein according to the present invention is characterized by heating and maintaining an aqueous dispersion of a protein-containing starting material while under an acidic condition with a plate-type heat exchanger using a liquid as a heat medium to make the aqueous dispersion substantially aseptic, and then subjecting the resulting aqueous dispersion to an action of a proteolytic enzyme. Thus the present invention makes it possible to mass-produce a hydrolyzed protein easily on an industrial scale.
Thus, the present invention provides a method for rendering a protein-containing starting material substantially aseptic, wherein said method comprises:
(a) heating an aqueous dispersion of a protein-containing starting material with a plate-type heat exchanger with a liquid heating medium, to obtain a heated dispersion; and
(b) maintaining said heated dispersion at an elevated temperature, to obtain a substantially aseptic dispersion,
wherein said aqueous dispersion is acidic during said heating and said heated dispersion is acid during said maintaining.
As the aqueous dispersion of the protein-containing starting material, a dispersion having a high concentration of preferably at least 10 g/dl, more preferably 10 to 50 g/dl or so, is used. The dispersion should be under an acidic condition at a time for the step of heating and maintaining (heating and retention steps). It is preferable that an acidic condition is employed not only in the heating and retention steps but also at the stage of preparing the dispersion. Further, with respect to the acidic condition in the heating and retention steps, the pH value is preferably between 3 and 6. Further, as the liquid heating medium used in the plate-type heat exchanger, hot water of 120 to 150xc2x0 C. is preferably used. Under such conditions, the protein-containing starting material is heated to 120 to 140xc2x0 C. (subject temperature, i.e., the temperature of the dispersion) and maintained at that temperature for 1 to 20 minutes or so, preferably for 5 to 15 minutes or so. Such a construction of the present invention makes it possible to prevent denaturation of the protein and perform complete sterilization of the protein-containing starting material dispersion having a high concentration.
A key feature of the present invention is that a uniform aqueous dispersion of the protein-containing starting material is heated and maintained under the foregoing specific conditions. By this characteristic, even when using a protein-containing starting material dispersion having a high concentration, the denaturation of the protein can be prevented and the dispersion can be made substantially aseptic, which leads to an improvement of the productivity and the stabilization of the quality.
Further, in the process for producing a hydrolyzed protein according to the present invention, a partially hydrolyzed protein can be used as a protein-containing starting material. The protein dispersion having a high concentration can be easily produced by previously lowering the molecular weight of the protein-containing starting material.